What is a word that describes someone who continues to do something that they think is good for themselves, but is really the opposite?

Question

I am looking for a specific word that describes how someone would continuously do something that they think is good for themselves, but is really the opposite. What I mean by that is: whenever you do something that you think is good, but it actually harms you. For instance, people who drink diet sodas; they THINK it's good for them, but it's really not. However, they continue to do it. In simple words, it is whenever someone thinks what they are doing benefits them, but it doesn't. Help?

To answer livresque, it's a word that described someone; an adjective. "My friend is a(n) [unknown word] because he wants to drink diet soda, but he doesn't know it's bad for him."

To answer DjinTonic, it's not quite misinformed, though it could be. It was a very philosophical term one of my teachers briefly explained to me, but I just can't remember what it was. Without trying to be condescending, I doubt anyone here will guess it, I'm asking for a very specific term, one that not many people have heard.

To answer HippoSawrUs, I'm thinking about this in the context of a complex topic, much more complex than diet sodas (I just used the analogy to simplify it). I can be about a topic that affects a large group of people. For instance, let's take the United States economy vs environment debate: Should the US focus more on its economy or more on its environment? Most people would argue for the latter, but they don't necessarily know the impacts that it would bring upon them. I guess that's what I'm trying to get at, people choose to follow or do something that they think (or sounds) good for them, but they are oblivious to the consequences. And before you say it, no, the word I'm looking for is not obliviousness.

To answer Old Brixtonian, it's not used as an insult much. Although obstinate is on the right path. I'll do more research surrounding that word.

To answer Xanne: refer to people in cults as a more serious example. They think their way is best, and they think their way is right, but in reality, well, you can answer that question.

To answer Elliot: technically, yes, but that is not the word I'm looking for. The definition I provided does not fit the exact definition of disagree, nor does it fall in those lines.

Answer 1

The word you are looking for is "DELUSIONAL" :

Delusional :
Believing things that are not true

Delusion :
A mistaken or unfounded opinion or idea

Eg : Your friend is Delusional , having the Delusion that "Diet Soft Drinks" are good.

In Extreme Cases , it may be a "Psychological Disorder" of some variety.

Eg : The President is Delusional , having the Delusion that Martian Aliens are roaming nearby trying to assassinate everybody.

Answer 2

Jokingly...consider Trumpian. But seriously: misguided

led or prompted by wrong or inappropriate motives or ideals

https://www.merriam-webster.com/dictionary/misguided

Answer 3

Counterproductive maybe? But that'd describe their actions more than themself.

Answer 4

Stubborn (Wiktionary) Refusing to move or to change one's opinion; obstinate; firmly resisting; persistent in doing something.

Bullheaded (Wiktionary) unreasonably stubborn.

Pigheaded (Wiktionary) Obstinate and stubborn to the point of stupidity.

Obdurate (Wiktionary) Stubbornly persistent, generally in wrongdoing; refusing to reform or repent.

Edit: If these words won't do, word combinations can be used such as "erroneously persevering fool", "imprudent in his/her ways" etc. I have added a few more words below.

Foolhardy (Wiktionary) Marked by unthinking recklessness with disregard for danger; boldly rash; hotheaded.

Foolish (Wiktionary) Lacking good sense or judgement; unwise.

Imprudent (Wiktionary) Not prudent; lacking prudence or discretion; indiscreet; injudicious; not paying attention to the consequences of one's actions